This invention is directed to a method and apparatus for controlling or determining the carbon content of a heat in a BOF vessel, and more particularly, to a method for determining the in-blow carbon content and the First Turn Down Carbon (FTDC), in low carbon steel BOF heats containing 0.06% or less carbon.
Users of flat rolled steel product demand low carbon grade steel because of its good formability properties. For example, in the automotive industry, such low carbon steel permits auto manufacturers to stamp and form complex automobile shapes without encountering steel spring-back after the foraging operations. This makes it necessary for steelmakers to accurately manage and control the carbon content of their BOF heats to produce a product having the proper metallurgical requirements.
In the BOF steehnaking process carbon saturated liquid iron is poured into the vessel along with various amounts of steel scrap. tligh velocity oxygen is blown into the BOF vessel at the surface of the molten steel bath where it reacts with the carbon to form CO and CO.sub.2. This reaction removes excess carbon in the steel bath and produces a finished product having the desired carbon content.
There are many BOF process control methods available to present day steelmakers. These controls range from sophisticated predictive models that are managed through the use of computers in combination with sensor instruments such as gas analyzers, thermocouples, load cells, etc.
In the past, various attempts have been made to control the carbon content in a vessel using flame drop measurements. One such past attempt is shown in U.S. Pat. No. 3,652,262 granted to Denis. This patent discloses using a sensor to detect infrared radiation emitted from a BOF vessel. The signal from the infrared sensor is processed to generate a curve representing a function of radiation intensity against time. In his patent, Denis compared his radiation curve with a decarburization curve generated by using readings taken from a first gas pickup used to measure the concentration of CO.sub.2 and CO in the off-gas of a BOF vessel, and from a second gas pickup used to measure the total gas output. He then compared the two curves and concluded that his time/radiation curve was useful in providing an instantaneous carbon reading during BOF steelmaking operations. However, if the two different graphs are compared, it can be seen that wide variations in predicted carbon levels occur between the off-gas curve and the radiation curve. Therefore, although Denis has provided some improvement in providing an instantaneous reading of carbon content during a heat, his patent shows a wide margin of error in his predicted carbon levels based upon his flame drop readings.
Additionally, in a study found in chapter fifteen entitled "BOF Control", of an Iron & Steel Society publication "BOF STEELMAKING" dated 1977, J. H. Cox, et al. teach that flame intensity is a function of the carbon in the bath. However, the authors also teach that carbon predictions, based upon flame intensity measurements, are not satisfactory for the more stringent present day needs.
Such beliefs have become widespread throughout the steelmaking industry. They have led steelmakers to use control strategies based on statistical, predictive-adaptive control models, or highly sophisticated control systems based on a continuous or periodic measurement of variables such as carbon, temperature, etc. (J. H. Cox, et al. "BOF STEELMAKING"). One such measurement process is based upon mass/temperature calculations to determine the carbon content of a BOF heat. It is well known that such mass/temperature calculations contain a margin of error, and they often lead to either overblowing or underblowing the BOF heats.
In instances where a heat is overblown various undesirable chemical reactions take place within the vessel. For example, in an overblown heat, the oxygen consumes an excessive amount of carbon and a steel product having an undesirable low carbon level is produced. The excess oxygen also reacts with the molten iron to form iron oxides. This reduces the iron yield of a heat. Overblowing a heat will also overheat the steelmaking vessel, cause premature wear on its protective refractory lining, and reduce the service life of the vessel.
In those instances where a heat is underblown, the heat may have to be reblown to further reduce the carbon level. This increases production time and cost, and causes excess refractory wear. The excess refractory wear is due to the iron oxides that are formed in the slag during the reblow. Iron oxides in the slag make the slag more corrosive to the refractory lining.
Another problem encountered with BOF control systems is dealing with the hostile environment adjacent the hot BOF vessel. The radiant heat emitted from a BOF vessel during the steelmaking process overheats sensitive electronic equipment located near the vessel and causes system failures. Dust and fume released from the vessel also settles on equipment located throughout the steelmaking shop, including the various sensor devices used to control the steelmaking process. The dust and fume causes control equipment sensors to become fouled and dirty, and results in poor readings and inaccurate metallurgical analysis.